Electric blanket shells must have spaced channels between the shell fabrics to receive electrical heating wires. These channels position the electrical heating wires and, accordingly, must be uniformly disposed across the length and width of the blanket shell (except at the top, bottom and sides thereof) in order to produce a uniform heating effect. On the other hand, the channels must prevent contact of adjacent wires or even close proximity thereof, since dangerous localized overheating may otherwise occur. Thus, the shell fabrics must be joined in an accurate and positive manner.
Woven blanket shells might be produced by weaving shell fabrics and sewing the fabrics halves together with conventional thread sewing to form these channels, but this technique would involve considerable hand labor to accurately sew the channels and insure continuous, unbroken stitching. Such a technique has never been economical. The conventional technique of forming the channels for electric blanket shells is by a special weaving technique. In effect, the two shell fabrics are woven at one time on a single loom with a pattern of common warp yarns which join or lash the two shell fabrics at spaced intervals to form the channels. While this technique provides a very predictable pattern of channels and is most positive in joining the shell fabrics, it does involve the relatively slow weaving process.
Needled blankets (i.e., non-woven) have the essential strength and wear characteristics of woven blankets, and the needling technique can produce blankets at rates of 30 to 50 times faster than the weaving process. However, the needling technique cannot form channels, as can the special weaving technique. Thus, if electric blanket shells are made of needled fabrics, the needled fabrics must be joined so as to form channels with accuracy and positiveness for the same reasons noted above. Also, the practical thickness of conventional needled fabrics is greater than that of woven fabrics and the resulting total thickness of a needled fabric blanket shell is too great for comfort in use. In view of the foregoing, needled fabrics have not been practical for producing electric blanket shells.
In view of the much greater production speed of needled fabrics, it would, of course, be most desirable to provide means of accurately and positively joining needled fabrics to produce channels for an electric blanket shell. In this regard, it is known that non-woven fabrics may be joined by adhesion bonding, i.e., the application of glues, or by fusion bonding, i.e., the application of heat as by hot gas, flame and conduction (e.g., a heated iron). Fusion bonding considerably reduces the strength and fibrous nature of the fabric in the vicinity of the fusion bond, and while adhesion bonding preserves the strength and fibrous nature of the fabric, the practical problems of handling glues make it difficult to accurately, predictably and positively join such fabrics
An alternative means of heating to cause fusion bonding is broadly known as vibrational heating. This technique involves the creation of a relatively high frequency vibration in a driver and this vibration is directed by a horn in physical association with the driver to localized parts of the substrate to be heated. A rigid element, called an anvil, is disposed on the opposite side of the substrate and vibrations induced in the substrate cause frictional heating. This vibrational energy can be in either the sonic or ultra-sonic frequency ranges. The technique has the advantage that the heat produced can be closely controlled and can produce predictable an localized heating. Vibrational heating has been used to weld thermoplastic sheets together. Localized heating of the sheets of thermoplastic along a narrow, relatively continuous band causes the thermoplastic sheets to be melted and then resolidified together.
This process works well with relatively high density, homogeneous, solid substrates, such as plastic sheets, because the welded (melted and resolidified) substrates are essentially the same as unwelded substrates in regard to the character and properties. However, with relatively low density, non-homogeneous substrates, such as textile fabrics, the process presents serious problems. A textile is considered to be non-homogeneous and relatively low density material in that it contains a large percent of voids between fibers.
It can be appreciated that a textile derives much of its character and physical properties from the spatial configuration of the fabrics and the interaction between fibers, while a plastic sheet derives much of its character and physical properties simply from the mass of the plastic therein. Thus, contrary to welding plastic sheets, any melting of fibers in a textile fabric tends to severely reduce the basic character and physical properties of fibers adjacent to a fusion bond. Nevertheless, vibrational heating has been used to bond textile fabrics and this technique has been referred to as sonic or ultra-sonic sewing or seaming, although the method really involves fusion bonding. Conventional apparatus and methods of such sewing and seaming are illustrated in U.S. Pat. Nos. 3,666,599, issued May 30, 1972, and 3,734,805, issued May 22, 1973. The technique can also be used for placing patterns on textile materials, and U.S. Pat. No. 3,733,238, issued on May, 15, 1973, is representative of that art. Since this art is well known, the details of the apparatus and the methods will not be repeated herein and the aforenoted U.S. patents are incorporated herein by reference and relied upon for those known details.
As noted above, the localized heating produced by the vibrational energy causes fusion of the fibers of the textile fabric and this fusion action alters the molecular structure of the fibers and therefore degrades the physical properties and character of the fibers in and adjacent to the fusion area. Under the circumstances, localized weakening of the bonded fabric takes place in and around fusion bonds. When a pattern of such fusion bonds exists across a dimension of a bonded fabric, then that pattern will tend to form a corresponding pattern of weakened areas. More specifically, a pattern of fusion bonds may define a weakened line or "tear-line" in the plane of the fabric. In woven textile fabrics, this undesired "tear-line" effect is somewhat mitigated by the very nature of the woven fabric itself. Thus, the tight, twisted arrangement of fibers within the yarns making up a woven fabric produces an extremely high degree of interfiber friction. Additionally, the systematic arrangement of these twisted yarns making up the warp and filling of the woven fabric, along with the interfiber friction of the twisted yarns, produces a very high degree of fabric integrity. This high degree of integrity is such that even if many of the fibers, or even yarns, are substantially degraded by the fusion bonding, the remaining interfiber friction and systematic arrangement still provide quite high fabric integrity and, correspondingly, the strength of the fabric which remains is sufficient for many purposes.
In contrast, the fiber arrangement in a needled fabric is far more random rather than systematic, i.e., as compared with the twisted and precise pattern of a woven fabric. Such arrangement is, therefore, less efficient in producing interfiber friction. The needling process results in a complex fiber entanglement which give rise to the interfiber friction of a needled fabric and if this entanglement is degraded in a bonding operation, the essential strength providing feature of the needled fabric is seriously degraded. Thus, when a pattern of fusion areas exist across a dimension of needled fabrics, then that pattern will tend to form a corresponding pattern of pronounced weakened areas and the "tear-line" effect becomes most serious. As a result of the foregoing, sonic or ultra-sonic bonding of needled textile fabrics has not been generally accepted where the bonded fabric must provide a strong joint and can not exhibit a "tear-line" effect.
It would, however, be most advantageous to provide a method of fusion bonding needled textile fabrics to form electric blanket shells wherein the foregoing difficulties are essentially obviated.